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Date:   Fri, 29 May 2020 21:51:26 +1000
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Eric Biggers <ebiggers@kernel.org>,
        Stephan Mueller <smueller@chronox.de>
Subject: Re: [RFC/RFT PATCH 0/2] crypto: add CTS output IVs for arm64 and
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On Fri, May 29, 2020 at 10:20:27AM +0200, Ard Biesheuvel wrote:
>
> But many implementation do not return an output IV at all. The only
> mode that requires it (for the selftests to pass) is CBC.

Most modes can be chained, e.g., CBC, PCBC, OFB, CFB and CTR.
As it stands algif_skcipher requres all algorithms to support
chaining.
 
> For XTS, we would have to carry some metadata around that tells you
> whether the initial encryption of the IV has occurred or not. In the

You're right, XTS in its current form cannot be chained.  So we
do need a way to mark that for algif_skcipher.

> CTS case, you need two swap the last two blocks of ciphertext at the
> very end.

CTS can be easily chained.  You just need to always keep two blocks
from being processed until you reach the end.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt